1. Field of the Invention
Embodiments of the present invention generally relate to a plasma enhanced chemical vapor deposition (PECVD) method and apparatus.
2. Description of the Related Art
Photovoltaic (PV) or solar cells are devices which convert sunlight into direct current (DC) electrical power. A typical PV cell includes a p-type silicon wafer, or substrate, typically less than about 0.3 mm thick, with a thin layer of an n-type silicon material disposed on top of the p-type substrate. When exposed to sunlight, the p-n junction generates pairs of free electrons and holes. An electric field formed across a depletion region of the p-n junction separates the free holes from the free electrons, which may flow through an external circuit or electrical load. The voltage, or photo-voltage, and current generated by the PV cell are dependent on the material properties of the p-n junction, the interfacial properties between deposited layers, and the surface area of the device.
Conventional methods of forming p-n junctions typically include depositing n-type and/or p-type layers via radio frequency (RF) plasma enhanced chemical vapor deposition (PECVD) processes which utilize RF frequencies of less than about 30 MHz. However, conventional RF-PECVD processes often exhibit poor plasma density and limited control over deposition rate, resulting in the deposition of semiconductor layers which lack uniformity and exhibit unsatisfactory doping efficiencies. Moreover, attempting to increase doping efficiencies of conventional RF-PECVD processes typically increases the degree of plasma, charge, and thermal damage in the resulting films, resulting in poor interface passivation characteristics and decreased device performance.
As the foregoing illustrates, there is a need in the art for more effective techniques for depositing solar cell films.